To impart motion of a driving member to a driven member, the driving and driven members have to be latched and held without relative rotational motion between them. For example, in a cam-and-follower arrangement, cam motion is imparted to a valve-actuating rocker arm by a cam follower. Here the driving member is the cam follower and the driven member is the rocker arm. The rocker arm is pivoted to ground at one end and actuates a valve against a valve spring at its opposite end. A locking pin may be used to selectively impart motion from a driving member to a driven member. However, an accidental drop in hydraulic-holding pressure may cause the locking pin to instantaneously retract to an unlatched position.